This invention relates to apparatus for and a method of forming ductile metal sheets into shaped bodies. It is particularly although not exclusively concerned with the forming of so called superplastic metal alloys which under appropriate conditions of temperature and pressure exhibit high ductility and an ability to flow.
Many superplastic alloy sheets, particularly those with an aluminium base can at least in their superplastic condition, be formed by the application of a differential air pressure thereto. Simple female forming in which a sheet is deformed into an open mould is suitable for shallow, smoothly rounded shapes but may result in bodies having unacceptable variations in wall thickness. For deeper bodies or those of more complex shape particularly those having sharp corners or angular shapes it is usual to form the sheet over a male mould either in a single operation or as the last step of a multi-stage operation.
In a simple male forming operation the male mould is conveniently carried on a platen of a forming machine which platen must have a peripheral wall of greater depth than the mould to enable the metal sheet which is to be formed over the mould to be initially clamped around its periphery in spaced relationship to the mould. With this arrangement it is necessary for the periphery of the mould to be spaced significantly inwardly from the wall in order to ensure a smooth flow of the sheet material around the base of the mould. This is wasteful of material since the original flat sheet must be significantly larger than the mould.
It is also known, for example in the so-called "snap back" technique to clamp a sheet of superplastic metal alloy around its periphery, hold the sheet at a forming temperature, form a bubble in the sheet by applying a differential air pressure thereto, advance a male mould into the cavity of the bubble and reverse the pressure differential to form the sheet against the mould. If air under pressure is used to form the bubble such an operation requires establishing a sealed chamber on each side of the sheet and moving a carrier for the mould within one of the chambers. Forming machines for this purpose usually have a mould carrier in the form of a movable platen and this inevitably results in sealing problems where the drive for the platen passes through its associated chamber wall.
It is accordingly an object of the present invention to provide an improved press for use in forming ductile metal sheets into shaped bodies which is particularly flexible in operation. A further object is to provide an improved method of forming ductile metal sheets into shaped bodies using a "snap back" technique.